1. Field of the Invention
The present invention relates to a semiconductor device and a method of manufacturing the semiconductor device, and to, for example, a metal oxide semiconductor field effect transistor (MOSFET).
2. Description of the Related Art
A factor that determines the threshold voltage for a MOSFET is the concentration of impurities in the area (hereinafter referred to as a channel area) between source/drain areas. In general, the threshold voltage increases as the concentration of impurities increases. It is known that the off leakage current can be reduced by increasing the threshold voltage.
However, an increase in the concentration of impurities in the channel area may increase the leakage current among the source/drain area and the gate electrode and the semiconductor substrate, more specifically, between the source/drain area and the channel area. If a MOSFET with a large leakage current from this part is used in each memory cell of, for example, a DRAM (Dynamic Random Access Memory), the pause characteristic (refresh characteristic) of the memory cell may be degraded. That is, the time for which the memory cell can hold data may decrease. Thus, there is a tradeoff between the off leakage current and the leakage current flowing between the source/drain area and the channel area.